A converter (or commutator) is conventionally used for polarity reversal of a phase of the electromagnetic energizing field of an electric motor in the area of the zero crossing point. The zero crossing point is taken to be the position of the phase in which the phase is aligned in parallel to the stator field of the motor, so that no torque is exerted on the rotor of the motor.
With a modern electric motor, especially as used in drive systems of electric vehicles, commutation is mostly undertaken using power transistors.
For the commutation of such an electronically commutatated motor a full electrical cycle, i.e. a full rotation of the energizing field by 360° is subdivided into similar shaped zones (or “states”). At the beginning of each zone the power transistors can be switched on or off for commutating a phase. The state of the phase is then retained until at least the end of the zone, but can optionally be overlaid by a pulse width modulated (PWM) signal.
The division of the full cycle into zones defines a discrete number of possible commutation angles of the phase or of each phase. In this case that part of the full cycle during which the phase or each phase is activated, i.e. energized, is designated as the commutation angle. For a division of the full cycle into twelve equal zones, the commutation angle can for example be 180°, 150°, 120°, . . . . Previously an electric motor has frequently been operated with a fixed commutation angle.